A commonly used input stage in operational amplifiers is a differential-input amplifier of a sort known as the "long-tailed-pair." Differential input voltage is applied between the input electrodes of two transistors, the common electrodes of which connect to a common node and thence to a "tail connection" which includes a constant current sink (or source) that establishes the combined currents of the long-tailed-pair transistors. At least one of these transistors has its output electrode connected through a direct current conductive load to a first operating potential connection. The voltage appearing across this load responsive to differential input voltage is remote from a second operating potential. Level shifter circuitry is used to translate this response voltage closer to the second operating potential to facilitate its being applied to an ensuing amplifier stage, usually the output stage of the operational amplifier.
This basic operational amplifier configuration is used especially in monolithic integrated circuitry using bipolar transistors, where one conductivity type transistor (usually NPN) has good high-frequency response and the other conductivity type (usually PNP) has poor high-frequency response. Transistors of the former conductivity type are typically employed for all signal-processing functions, and transistors of the latter conductivity type are typically employed for establishing quiescent biasing conditions. The operational amplifier then exhibits overall frequency response limited only by the better-high-frequency-response transistors.
A concern in operational amplifier design is to minimize common-mode input current demands. Accordingly, the two transistors used in the long-tailed-pair configuration are sometimes field effect types, but more often they are each a composite transistor. For example, the composite transistor may be the well-known Darlington connection of two bipolar transistors. An operational amplifier is often arranged so as to apply signal with a direct potential component to the non-inverting terminal of the long-tailed-pair input stage, with voltage feedback from the operational amplifier output terminal being used to adjust the potential applied to the inverting terminal of the long-tailed-pair input stage. Ideally, presuming the transistors in the long-tailed-pair to be identically similar, adjustment is made to balance the quiescent current flows in the long-tailed-pair transistors when zero differential input voltage appears between the non-inverting and inverting input terminals. This provides for the lowest input voltage offset error in the long-tailed-pair input stage.
If the level shifter stage demands quiescent input current that disrupts the balance of the quiescent current flows in the long-tailed-pair, the voltage feedback from the operational amplifier output terminal will introduce a differential-input offset voltage to support this input current demand. This offset is accompanied by an undesirable loss in dynamic range wherein large differential input signals are asymmetrically clipped. The invention concerns a level shifter the quiescent input current demand of which is substantially zero-valued and consequently does not unbalance current flows in the long-tailed-pair.
This level shifter puts to dual use a first current mirror amplifier also employed to provide a constant current load to the output electrode of one of the long-tailed-pair transistors. In this specification and the claims that follow it, current mirror amplifiers will be considered to operate as transistors with predetermined common-emitter forward current gains, or h.sub.fe 's, and to be of conductivity types corresponding to such transistors. The first current mirror amplifier, employed to provide a constant current load to the output electrode of a given one of the long-tailed-pair transistors is of a conductivity type opposite to that of those transistors, has its output terminal connected to the output electrode of the given one of the long-tailed-pair transistors, has its common terminal connected to a first operating potential connection and has its input terminal connected to a second operating potential connection via a current sink (or source) related to that in the tail connection of a long-tailed-pair.